1. Field of the Invention
The present invention relates to device and method for detecting a pilot PN (Pseudo-Noise) offset in a cordless telephone system, and more particularly, to device and method for detecting a pilot PN offset in a cordless telephone system having a cellular handset and a home station connected to PSTN (Public Switching Telephone Network), which can detect all pilot PN offsets within a service region of the home station for detecting a pilot PN offset which are not in use by other subscribers.
2. Description of the Related Arts
In general, the cordless telephone system, operative within a frequency band of 800xcx9c900 MHz, is provided with a handset for talking thereto and hearing therefrom and a home station for charging the handset when the handset is placed thereon and offering pre-programmed additional services. The home station in general is stationed at a certain place for power supply and connected to PSTN. In transmission of a signal using such a cordless telephone system, the signal is transmitted from the handset to the PSTN through the home station. Opposite to this, in reception of a signal using such a cordless telephone system, the signal received from the PSTN is transmitted to the handset through the home station. In a background art cordless telephone system, there should be a channel of a certain frequency set-up between the home station and the handset in the cordless telephone system for either calling a subscriber or receiving a call from a subscriber using the handset.
The channel is set-up in the background art cordless telephone system according to the following steps.
A controlling part in the cordless telephone system determines the present mode if a standby condition is established by a power supply to the cordless telephone system. If the present mode is either a reception or transmission mode, one channel from preset plurality of channels is locked in the order of a preset sequence, and a RSSI (Received Signal Strength Indicator) of the locked channel is detected. If the detected RSSI is below a preset level, the channel is determined to be a channel none of subscribers around the home station is using at the moment, and either the transmission or reception mode is carried out through the locked channel. On the other hand, if the detected RSSI is above the preset level, determining that the channel being of a channel one of the subscribers around the home station is using at the moment, the next channel is locked, and the aforementioned process is repeated in succession. If the RSSIs detected from all the channels in the preset frequency band are above the preset level, which indicates that all the channels are occupied by other subscribers around the home station, the fact is informed to the handset of caller. However, the background art cordless telephone system has a problem in that the system reacts so sensitively to interferences of electromagnetic waves generated in electronic appliances around the home station as to determine that channels that are not used by other subscribers are misunderstood as being used by the other subscribers, which provide only limited channels to the home station. Under such a situation, if plural subscribers around the background art cordless telephone system try calling within the same time band, a subscriber trying relatively later within the time band may not have a channel assigned to the subscriber. Further, there may be crosstalk during use because of a misunderstanding that a channel in use actually is considered not in use due to the electromagnetic interference.
Another problem in the background art cordless telephone system is that a number of the channels available for being set up between the home station and handset is limited because of its frequency division multiple access system. Therefore, the aforementioned problems may occur more frequently in cases when many offices are present on the same floor of a building, or depending on locations and service areas of the home stations of the cordless telephone system in apartments. In order to overcome such problems of the background art cordless telephone system, one suggestion is application of a cellular network of a CDMA system between the home stations and the handsets in the cordless telephone system. In other words, the handset is used as a mobile station in a cellular mobile communication system, and the home station is made to serve as a base station. However, such a suggestion has the following problems. As shown FIG. 1A, in designing a cellular mobile communication system, a PN sequence synchronous to a GPS (Global Positioning System) time, a standard time, is divided into specific intervals (PN offset=0, 64, or 256) and the PN offsets of the PN sequence are assigned to the base stations such that the PN offsets are not overlapped between adjacent stations for distinguishing base stations BS1, BS2 and BS3 and their stations from one another. However, the same principle cannot be applied to the home station of a cordless telephone system because the system has been wide spread, which causes cases when the PN offsets are overlapped when many offices are present on the same floor in a building or depending on locations of the home stations in places like an apartment. That is, as shown in FIG. 1B illustrating an example of pilot PN offsets for home stations in a cordless telephone system, service regions may be overlapped between a few home stations HS1, HS2 and HS3 as well as the pilot offsets (for example, PN offset=0, 64, 256) used.
Accordingly, the present invention is directed to device and method for detecting pilot PN offsets in a cordless telephone system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide device and method for detecting a pilot PN offset in a cordless telephone system, in which all pilot PN offsets within a service region of the home station can be detected for a detecting pilot PN offset which are not in use by other subscribers and providing the detected pilot PN offset to a respective handset.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the device for detecting a pilot PN offset in a cordless telephone system includes pilot PN sequence generators each for generating a pilot PN sequence for tracking a synchronization of a pilot channel signal for itself, despreaders for despreading the pilot PN sequences provided from the pilot PN sequence generators and channel data signals detected within an overlapped service radius to one another, an accumulator for accumulating datastreams of pilot channels provided from the despreaders for a certain period, an energy value calculator for calculating an energy value of a signal from the accumulator for turned on time and delayed time, a storage region for storing at least one different pilot PN offsets detected within a set service region and threshold values of energy values, and a controlling part for determining different pilot PN offsets existing in an overlapped service region using the pilot PN offsets stored in the storage region and energy values of the PN offsets, detecting a pilot PN offset not in use, and providing an optimal pilot channel signal. In a preferred embodiment, the storage region comprises at least one register and storage area.
In other aspect of the present invention, there is provided a method for detecting a pilot PN offset in a cordless telephone system, including the steps of initializing a storage region, measuring an energy value of a pilot channel signal for one reference offset, obtaining all different pilot PN offsets detected within an overlapped service radius, comparing measured energy values to a threshold value of the energy values of the PN offsets in a predetermined sequence, constructing a database using a present pilot PN offset and the energy value of the present pilot PN offset in case the measured energy value exceeds the threshold value of the energy value of the PN offset, and selecting one of pilot PN offsets which are not included in the database when it is determined that the comparing step has been completed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.